It has heretofore been widely practiced to coat a cationically electrodepositable paint by electrodeposition and curing with heating, followed by further coating an intercoat paint or a topcoat paint and then curing with heating.
However, in view of reduction in the number of steps of painting, economization of resources and prevention of environmental pollution, there has been a keen desire to develop a method for forming a paint film by a two-coat-one-bake process in which a surface coated with a cationically electrodepositable paint is further coated with an aqueous intercoat or topcoat paint without curing with heating the previously coated surface of the electrodepositable paint, and then the both paint films are heated to cure them simultaneously. However, up to present none has been put into practical use.
For example, when after it is coated with an organic solvent type paint, a coated surface of an uncured, cationically electrodeposited paint is heated so that the both painted films can be cured, the uncured cationically electrodeposited paint-coated surface is swollen or corroded with an organic solvent contained in the organic solvent-type paint, resulting in that depressions and protrusions, wrinkles or shrinks tend to occur in the coated surface after the curing with heating.
Alternatively, when an aqueous paint composed mainly of a polyester resin and a fully etherated melamine resin is used instead of the aforementioned organic solvent type paint, defects such as depressions and protrusions, wrinkles or shrinks do not almost occur but a problem arises that basic substances or low molecular weight substances which vaporise from the uncured cationically electrodeposited film upon heating migrate in large amounts into the upper coated film to prevent its curing reaction, with the result that the upper film is not cured sufficiently. In addition, there is also a disadvantage that the aqueous paint film itself does not have an enough smoothness or distinctness of image gloss.
A primary object of the present invention is to solve the aforementioned drawbacks involved in the method for forming a multilayer paint film by painting a cationically electrodepositable paint and an aqueous paint by a two-coat-one-bake process.
Other objects and advantages of the present invention will be apparent from the following description.